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From: hshwang@astro.snu.ac.kr
Subject: Source-to-background count ratio
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Date: Wed, 13 Sep 2006 16:29:53 GMT
From: hshwang@astro.snu.ac.kr
To: xmmhelp@sciops.esa.int
Cc: hshwang@astro.snu.ac.kr
Subject: Source-to-background count ratio
Full_Name: Ho Seong Hwang
Submission from: (NULL) (147.46.30.123)



Dear XMM Helpdesk,

I'm trying to analyze the EPIC MOS1 data of A1060(obsid: 0206230101) and
to get the source-to-background count ratio (will be used as a scaling factor
for background subtraction) for outside the FOV region.
I'm using the background event file of Read & Ponman (2003).

However, the count ratio is not similar to the values of those found in Hayakawa
et al. (2006, PASJ, 58, 695). 
 ==>  count ratio for MOS: 1.132 (paper) vs. 0.0209315 (mine)

Since I'm new to X-ray data reduction, I'm afraid that I'm doing something
wrong. Do you have any idea? 
Thank you in advance.

Ho Seong Hwang.

Here's my commands:

epchain &
cp P0206230101M1S001MIEVLI0000.FIT objmos1Evts.FIT

1) A1060
1.1) Removing background flares
evselect table=objmos1Evts.FIT:EVENTS withfilteredset=yes \
   expression='PATTERN==0 &&(PI in [10000:12000])' \
   rateset=ratemos1.fits maketimecolumn=yes timecolumn=TIME \
   timebinsize=100 makeratecolumn=yes

dsplot table=ratemos1.fits x=TIME y=RATE withoffsetx=yes &

tabgtigen table=ratemos1.fits expression='RATE<=0.3' gtiset=gtimos1.fits

evselect table=objmos1Evts.FIT withfilteredset=true \
   filteredset=mos1_filt.fits \
   keepfilteroutput=true destruct=true \
   expression='(gti(gtimos1.fits,TIME) && (PATTERN<=12))'

1.2) Out of FOV count rate
evselect table=mos1_filt.fits withrateset=yes \
rateset=ratemos1flagbin62000.fits maketimecolumn=yes timecolumn=TIME \
timebinsize=62000 makeratecolumn=yes filtertype=expression \
expression='(PI in [10000:12000] && #XMMEA_16)'

dmlist ratemos1flagbin62000.fits data |more
--------------------------------------------------------------------------------
Data for Table Block RATE
--------------------------------------------------------------------------------
ROW    RATE                 ERROR                TIME
     1     0.02864516129032       0.000679719597 204893850.5069424212
     2               0.0005      8.980265101E-05 204955850.5069424212

==> Source count rate ~ 0.02864516129032

2) Background
2.1) Removing background flares
evselect table=E1_fm0000_M1.fits:EVENTS withfilteredset=yes \
   expression='PATTERN==0 &&(PI in [10000:12000])'\
   rateset=ratebgmos1.fits maketimecolumn=yes timecolumn=TIME \
   timebinsize=100 makeratecolumn=YES

dsplot table=ratebgmos1.fits x=TIME y=RATE withoffsetx=yes &

tabgtigen table=ratebgmos1.fits expression='RATE<=0.3' gtiset=gtibgmos1.fits

evselect table=E1_fm0000_M1.fits withfilteredset=true \
   filteredset=bgmos1_filt.fits \
   keepfilteroutput=true destruct=true \
   expression='(gti(gtibgmos1.fits,TIME) && (PATTERN<=12))'

source skycast.csh bgmos1_filt.fits $SAS_ODF/mos1_filt.fits
mos1BGcorrectsky.fits

2.2) Out of FOV count rate
evselect table=mos1BGcorrectsky.fits:EVENTS withrateset=yes \
rateset=ratebgmos1flagbin28900000.fits maketimecolumn=yes timecolumn=TIME \
timebinsize=28900000 makeratecolumn=yes filtertype=expression \
expression='(PI in [10000:12000])&& #XMMEA_16'

dmlist ratebgmos1flagbin28900000.fits data |more
--------------------------------------------------------------------------------
Data for Table Block RATE
--------------------------------------------------------------------------------
ROW    RATE                 ERROR                TIME
     1      0.0005995847751      4.554874096E-06 124394715.0144954622
     2      2.083044983E-05      8.489857541E-07 153294715.0144954622

==> Background count rate ~ 0.0005995847751

Therefore, Source-to-Background count ratio ~ 0.0005995847751/0.02864516129032 ~
0.0209315  ; Why is it so small?



Reply 1

Resend
From: Maria Santos-Lleo <xmmhelp@sciops.esa.int>
To: hshwang@astro.snu.ac.kr
Subject: Re: Source-to-background count ratio (PR#23165)
Date: Fri Sep 15 11:44:20 2006
Dear Ho Seong Hwang, 

After having read your question, I asked for some advice 
to the people working here in the background analysis. 
This is my answer with their comments already incorporated.

We would like to suggest you to use the new blank sky fields, 
see:

http://xmm.esac.esa.int/external/xmm_sw_cal/background/blank_sky.shtml

Apart from that, the procedure you follow seems correct
to us, with a few comments that are given here below. 

Please note that the source to background ratio
you get at the end is not so small as you though. It seems that
just by mistake, you computed the inverse of the number
you wanted !. Your number is background_to_source ratio, 
rather than source_to_background. The source_to_background 
is 0.0286/0.0006~47.7 (and not 0.0006/0.0286~0.02 as in your 
question). A source to background of about 50 is fine. 
Of course the value of this ratio depends a lot on the specific 
observation 

Last, we would like to recommend as well to avoid using 
such large values for the timebinsize parameters: why not 
reading the source and background rates directly from the 
plotted light curves?

Hope this helps

With best regards

Maria

Maria Santos-Lleo, 
XMM-Newton SOC 
User Support Group

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